It is often necessary to survey a bore hole in the earth to determine the exact path or location of the hole at all levels. For example, in the fields of oil and gas drilling and geological testing, it is necessary to correlate formations found at different depths in the bore hole, and to do so it is also necessary to know the spatial coordinates of all points along the bore hole. Since the drill bit typically wanders from a straight vertical path during the drilling of the hole, for bore holes of any appreciable depth the location cannot be predicted without specialized survey apparatus.
Numerous systems have been used in the prior art for providing survey data for bore holes. Generally, an instrumented pod is lowered into the bore hole and readings are taken by instruments within the pod and transmitted by wire or otherwise to the surface. Various types of inclinometers or accelerometers, gyroscopes, magnetic sensors and the like have been used to attempt to measure the inclination and direction, or azimuth, of the bore hole at different levels, so that a map may be made for the bore hole. While such systems have achieved a degree of success, in many cases problems with accuracy, cost of manufacture, and slow, time-consuming operation remain. For example, magnetic sensors, which are used in numerous systems for sensing the direction of the Earth's magnetic field within the bore hole to thereby provide a north reference, are inherently subject to potential errors in this environment. Iron-bearing geologic formations at different depths can cause erroneous readings, and of course the instrument cannot be used in the vicinity of ferrous casings, shafts, or other tools, thus creating special application problems. Accelerometers are potentially accurate and reliable devices, but alone cannot fully determine the spatial location and orientation of the instrument pod. Free directional gyroscopes and gyroscopes having multiple sensing axes have been used, but these are complex and costly, and in some cases have drift or precession problems which must be corrected for. Rate gyroscopes can be somewhat smaller and more reliable, but in the past they have been used together with motors and drive apparatus for rotating the rate gyroscope to thereby serve as a north direction finder. Such drive motors and apparatus add cost and complexity and take up valuable space within the instrument pod.